Common examples include any POS, point of sale, system. I've seen anything from book stores, computer stores, furniture stores, etc. based POS systems.
To start out, it's typically best to create an "inventory" file so that new items can be added to it, prices can be changed, etc. Your next step is to design rules for your program: is there going to be sales tax, is there a max/min purchase amount, should certain items only be allowed to be bought at x quantities, is there a price break with buying x amount of something. All things that you should consider.
Next, you need to design the physical interface. Do you want to have menus, or should the person type in the item# manually, the amount bought, etc. Should they be allowed to add things before checkout, or must they pay for everything as soon as it's rang in, should they be allowed to remove items in case they don't have the money or see a better deal on a similar item.
These programs can be as advanced or as simple as you want. You have to come up with your game plan before you can even attempt to write code. Don't worry, if you have a lot of ideas for it but once you start writing it you realize it's going to be too complicated to implement, remove it. But I'd say to stick with at least 80% of your original ideas so that you don't detract too much from your concept.
Once you start creating code, however, post back here with any errors or any ideas you're having a hard time implementing. Also, don't be worried if it takes several hours and even more lines of code before you physically have something to compile. A great program will take some time to write the classes used to make it good, and even longer to come up with the ideas behind each of them.
Please note, I don't know your skill level, and some of this stuff, depending on that, may be a little hard to implement. But if this is a school project, don't be afraid to learn a few things. No teacher will ever complain about self teaching, especially when I see what some of the teachers think of as teaching.
i really i appreciate this!
about my skills level i am doing my 3rd year in secondary school computer science,
i tried my ideas and you gave a really good support soon will show you when i done with some errors thanks